1. Field of the Invention
This invention lies in the field of geophysical measurements on the earth's surface for determining subsurface physical conditions in the earth.
This invention is more particularly concerned with making physical measurements at the surface of the earth for determining the position and movement of a flame front within a subsurface formation at a selected depth under the surface, the progress of the flame front being caused by continued supply of air through at least one central well.
This invention is still more particularly concerned with detection of the presence and passage of a flame front within a geologic formation under the surface of the earth, where the overburden rock is supported in part by a combustible hydrocarbon material. As the flame front progresses and moves past a selected point, the overburden will slump, thinning the formation, and causing a slight lowering of the earth's surface over the slumped portion, which will cause a slight tilting of the earth's surface, which is detectable by tilt meters.
2. Description of the Prior Art
In the prior art, experiments have been carried on for many years by oil companies and others, in which coal, tar sands, oil shales, and other combustible hydrocarbon materials have been burned, in situ, within their natural formations, at selected shallow depths in the earth. Many experiments have been carried out, using thermal, gravity, magnetic, and other measurements, to indicate the position of the flame front at any time, but without success.
Knowledge of the position of the flame front is very important in the management of the operation since it is desired that the flame front be moved outwardly in a selected manner so as to completely burn all the material within a selected area. Lack of knowledge may make it impossible to burn all the material and, therefore, the operation would be less efficient than it might be.
To date, there has not been any satisfactory method for monitoring the position and the progress of a flame front in an underground burning operation in a geologic formation.